Pressure-sensitive adhesive tapes have been used for more than half a century for a variety of marking, holding, protecting, sealing and masking purposes. Pressure-sensitive adhesive tapes comprise a backing, or substrate, and a pressure-sensitive adhesive. Pressure-sensitive adhesives are materials which adhere with no more than applied finger pressure and are aggressively and permanently tacky. Pressure-sensitive adhesives require no activation other than the finger pressure, exert a strong holding force and should be removable from a smooth surface without leaving a residue.
Adhering to skin presents challenges to adhesive manufacturers due to the inherent variability of the properties of skin. Adhesion to skin is dependent upon many factors. These factors include but are not limited to the environment in which the recipient is located. For instance, adhesion to skin will vary on the same person depending upon the humidity. If the same person were tested for skin adhesion using a given adhesive in different climates, different adhesion results would be obtained depending upon if the person were located in an arid versus in a humid environment. Skin varies from individual to individual. One person may have extremely dry skin to the point of eczema whereas another person may have oily skin. As well as varying from individual to individual, skin properties may vary on a given individual depending upon the location. For instance, skin located on a hand may be considerably drier than skin located on a back or face. Therefore, it is very difficult to manufacture a skin adhesive which is suitable for the environmental and for the individual variabilities.
Adhesive composition and performance is also dependent upon the intended use of the adhesive. Some uses require a gentle tape whereas other uses require an aggressive tape. If an adhesive is adhered to a sensitive area of the body a gentle tape is used. However, if it is critical that the adhesive remain adhered for an extended period of time or if the adhesive is adhered to an area which is very mobile, a more aggressive adhesive is used. The term "gentle" adhesive generally refers to an adhesive for which the adhesion does not substantially build over time. The term "aggressive" adhesive refers to an adhesive which has a substantial resistance to lifting or peeling.
Medical adhesives are generally used in wound dressings, surgical drapes, bandages and tapes. These items are generally constructed of a backing coated with an adhesive. A liner may or may not be used to protect the adhesive. The performance of the adhesive is in part dependent upon the occlusivity of the backing. Backings are generally categorized by their porosity into either nonocclusive or occlusive backings. When nonocclusive backings are used to prepare bandages or the like for medical applications the resulting bandage typically does not adhere well to skin over extended time periods. This probably occurs because the bandage cannot release water vapor which causes retention of moisture and in turn causes the adhesive to lift from the skin.
Conformability and cohesiveness are two inversely related properties which are each important to consider when preparing or selecting adhesives for medical applications. It is desirable for a medical adhesive to conform to the terrain of the skin to which it is adhered. This enhances comfort to the wearer and also ensures a higher initial adhesion to the skin because the adhesive is able to flow into the skin's topography. However, if an adhesive is too conformable it may lack the necessary cohesiveness necessary to remove the dressing with the adhesive intact. If an adhesive lacks cohesive strength the adhesive on a bandage may split upon an attempt to remove the bandage leaving some adhesive residue adhered to the skin and some adhesive removed along with the bandage backing. This is unacceptable to most medical professionals and patients.
Pressure-sensitive adhesives require a delicate balance of viscous and elastic properties which result in a four-fold balance of adhesion, cohesion, stretchiness and elasticity. Pressure-sensitive adhesives generally comprise elastomers which are either inherently tacky, or elastomers or thermoplastic elastomers which are tackified with the addition of tackifying resins. They can be coated in solvent or as water-based emulsions to reduce the material viscosity to a level that is easily applied to a substrate of choice.
Generally, when additives are used to enhance properties of pressure-sensitive adhesives they are required to be miscible with the pressure-sensitive adhesive or to have some common blocks or groups to permit homogeneous blends to form at the molecular level. Pressure-sensitive adhesives have been modified to extend their applicability into new areas. Tackified thermoplastic elastomers have been dissolved in acrylic monomers and subsequently cured. Tackified thermoplastic elastomers have also been added to polymerized acrylic pressure-sensitive adhesives in solvent where each component contains a common segment to permit compatibility. Natural rubber has been added to polymerized acrylic pressure-sensitive adhesives in solvent and subsequently thermally cured. The general purpose is to combine the high shear properties of elastomers with the high tack performance of acrylics to achieve adhesion to both polar and nonpolar surfaces. Further improvements and better balance of properties continue to be sought.